Israel has lost the confidence of the West, Phillip Hammond has admitted, as news emerged of the bombing of another UN school.
At least 15 Palestinians were killed and 90 injured when shells hit the school, which was housing families who had followed Israeli instructions to leave the neighbourhoods they were attacking.
The attack came amid at least 40 Palestinian deaths overnight, as Israel ignored growing international outrage and conducted its heaviest bombing operation in the 23-day campaign.
"We have urged Israel from very beginning to act with proper regard to humanitarian law and to take greatest care to avoid any unnecessary civilian casualties," the foreign secretary said.
"I've explain to Israeli ministers and the prime minister that western public opinion - that was sympathetic to Israel over rocket attacks - is rapidly turning against Israel because of the scale of action going on in Gaza.
"Israelis have to understand that while they are defending their security they're also undermining the support for Israel that exists in the West."
The comments are the harshest from Hammond since the start of the conflict, during which he has tried to strike a centrist tone which would not offend Israel.
They are noticeably more robust than statements from David Cameron, who even yesterday was placing the blame for the conflict at the door of Hamas.
But it still pales in comparison to the international reaction, which has become aggressively anti-Israeli following weeks of images of dead families and homes reduced to rubble.
Chile and Peru recalled their ambassadors to Israel yesterday.
"Chile observes with great concern and discouragement that the military operations – which at this point appear to be a collective punishment to the Palestinian civil population in Gaza – don't respect fundamental norms of international humanitarian law," its foreign ministry said.
The international condemnation has not affected Israeli public opinion, however, with 95% of Israeli Jews feeling the assault is justified.
"What is happening in Gaza is a humanitarian catastrophe, the world is appalled by what it's seeing and is united in demanding of both sides an immediate and unconditional ceasefire," Hammond told the Today programme.
Hammond insisted that Israel "must be proportionate" in its Gaza operation, but refused to say whether he believed it was currently being disproportionate.
Admitting the Israeli action was disproportionate would be equivalent to saying it was breaking international law.
A UNHCR spokesperson told the programme: "These are entirely civilian populations seeking shelter. They have run from their homes and now they're under attack in UN buildings which are supposed to be protecting them."
Over the last 23 days, 1,240 Palestinians have been killed. On the Israeli side, 53 soldiers and three civilians have died.
A spokesperson for the Israeli Defence Force (IDF) said they were investigating the claims that the school had been struck, but analysts have increasingly lost confidence in its account of attacks following the bombing of a separate UN school last week.
Fifteen people died and 200 were injured during the strike, but the IDF said that only an empty "errant" shell had landed in the school playground.
UN testimonies said numerous shells fell within the vicinity of the school and journalists on the scene reported debris consistent with mortar rounds.
Meanwhile, the UK stepped up its humanitarian response to the crisis with another £3 million being sent to Gaza.
The funds are enough to provide emergency food for over 300,000 for a month, officials said.
It brings the total UK contribution to £10 million since the crisis began.
"We urgently need to stop the bloodshed: we continue to call for an unconditional and immediate humanitarian ceasefire to prevent any more needless suffering," international development secretary Justine Greening said.
"The situation in Gaza is dire."